In conventional surveillance and broadcast video systems, where many analog video sources are required to be combined or multiplexed for transmission over a single link, whether it be fiber or copper cable, the video inputs to the combining device typically have to be in analog form. The conversion from analog to digital is done inside the combining device, where the analog to digital converters can each be clocked by the same clock edge. This is shown in FIGS. 1C and 1D.
It will be evident to one skilled in the art that in order to combine, interleave or otherwise multiplex digital signals, into a single stream, the digital samples or in this case, pixels, must be precisely aligned in frequency and phase, as each sample must occupy a predetermined position in the stream. This necessitates that the analog to digital conversion for each signal to be multiplexed must be done at the same physical location with the same clock in order to minimize time skew between the digital samples or pixels, of each conversion. The requirement that the multiplexing device have analog inputs also imposes other restrictions, such as that the sources must not be more than approximately 30 meters from the multiplexer, in order to minimize high frequency roll-off due to cable losses. Connecting the sources to the multiplexer via coaxial or twisted-pair cable has other disadvantages, such as the size and weight of the cable, or problems arising from having a direct electrical connection between source and multiplexer. Some applications may require complete electrical isolation for safety reasons.
One may try to circumvent these problems, if long distances or electrical isolation are required, by using a fiber optic link to connect the video source to the multiplexer, but this link would either have to be analog intensity or FM modulated, which can introduce noise or distortion, or convert the video to digital form and then back to analog again, for input to the multiplexer, which is expensive and involves extra conversion processes which can also introduce noise or distortions. Some multiplexers do accept digital inputs, but if the sample rate of the source differs in frequency from the master clock frequency of the multiplexer, a FIFO frame buffer memory is required for each input. This is needed to change the frequency and phase of the source's digital samples to those of the multiplexer. But it can add greatly to the cost. This memory will also cause frames of video to be either dropped or repeated, depending on the relative frequencies of the input and master clocks, in order to prevent the memory from overflowing or underflowing. This repeating or dropping of frames can cause motion artifacts in the video image. A smaller, less expensive FIFO cannot be used because underflow or overflow would occur more rapidly, and would necessitate the dropping or repeating of individual pixels within a video frame, which would be more visually upsetting to the user, and cause a loss of synchronization in the video monitor, because the length of the video frame would be altered by the repetition or dropping of pixels.
Still other system designs may circumvent this problem by keeping the video in digital form, but packetizing it in asynchronous transmission protocols that add extra code words and/or start/stop bits to the transmitted stream that allow the data to be recovered by a receiver whose clock is not referenced to the source. This adds overhead in the form of increased bandwidth requirements for the communications link, complexity and cost. The increased bandwidth requirements can be addressed by resorting to video compression, but this can result in loss of quality, which can be in the form of motion artifacts or loss of fine detail in the image. This can be a disadvantage in a surveillance system when one is trying to capture an image such as a face or a license plate number. There is also the increased cost resulting from the compression and decompression circuitry.
Conventional surveillance or broadcast fiber optic video links also lack a built-in video test signal generator with multiple patterns at the source for performance verification and various system alignment tasks. Often, if a test signal is present, it is usually within the source itself, and limited to one pattern such as color bars. This is insufficient for all the tests that might need to be performed on the system.
An example of a conventional system is given in FIGS. 1C and 1D. The increased system complexity and additional wiring and interconnects are shown. The video, audio and data signals originate at the camera 105 in analog format and are interfaced to the fiber optic transmitter 202 via electrical cables 104. The traditional fiber optic transmitter 202 interfaces with a fiber optic cable 101 to a fiber optic receiver 203. The receiver decodes the video, audio and data signals from the digital domain back to the analog domain. A plurality of fiber optic receivers interface to a video and audio multiplexer 207 and a data multiplexer 201 via a complicated cabling arrangement including video cables 206, audio cables 205 and data cables 204. In most applications the data multiplexer requires at least two fiber optic cables 200 for the data transmission. The video and audio multiplexer requires a third fiber optic cable 101. The control room or receive end of the conventional system requires a video and audio demultiplexer 100 and a data demultiplexer 208.
Current systems also exhibit significant latency between the video image and the camera pan, tilt and zoom controls. Due to the latency, the user has difficulty controlling the pan, tilt and zoom functions of the camera system. The user executes a command to move a camera, but the system exhibits significant delay before movement is preserved on the video monitor. The latency causes the user to overcompensate all camera movements.
As is apparent from the foregoing, a need exists for an improved fiber optic communication system, suitable for simultaneous transport of multiple broadcast or surveillance video streams from distant sources, that is able to maintain a digital format, but in uncompressed form, from input to output, and that can provide a means for testing and performance validation as well as source identification.